The present embodiments relate to wireless communications systems and, more particularly, to Multiple-input Multiple-output (MIMO) communication with Per Group Rate Control (PGRC).
Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (CDMA) which includes wideband code division multiple access (WCDMA) cellular communications. In CDMA communications, user equipment (UE) (e.g., a hand held cellular phone, personal digital assistant, or other) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. WCDMA includes alternative methods of data transfer, one being frequency division duplex (FDD) and another being time division duplex (TDD), where the uplink and downlink channels are asymmetric for FDD and symmetric for TDD. Another wireless standard involves time division multiple access (TDMA) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into time slots so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a time slot in the time period, and that time slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to time slots.
Referring to FIG. 4, there is a wireless communication system or the prior art including a transmitter and user equipment 450-454. The transmitter includes a separate buffer 440-444 for each respective user equipment 450-454. Data from these buffers is applied to serial-to-parallel converter circuit 410. The serial-to-parallel circuit 410 converts the serial data streams into parallel data words which are then applied to modulation code scheme (MCS) circuits 400 and 404. The modulation code scheme circuits 400 and 404 transmit the signals via respective antennas 402 and 406 to user equipment within the wireless system. For example, a signal 462 from antenna 402 is transmitted to UE 1 450. Likewise, a signal 468 is transmitted from antenna 406 to UE 3 454. Antennas 402 and 406, however, also transmit respective interference signals 466 and 464. These interference signals degrade the intended data signal at the user equipment.
Wireless communications are also degraded by the channel effect. For example, the transmitted signals 462 and 468 in FIG. 4 are likely reflected by objects such as the ground, mountains, buildings, and other things that it contacts. Thus, when the transmitted communication arrives at the receiver, it has been affected by the channel effect as well as interference signals. Consequently, the originally-transmitted data is more difficult to decipher. Various approaches have been developed in an effort to reduce or remove the channel effect from the received signal so that the originally-transmitted data is properly recognized. In other words, these approaches endeavor to improve signal-to-interference+noise ratio (SINR), thereby improving other data accuracy measures (e.g., bit error rate (BER), frame error rate (FER), and symbol error rate (SER)).
One approach to improve SINR is referred to in the art as antenna diversity, which refers to using multiple antennas at the transmitter, receiver, or both. For example, in the prior art, a multiple-antenna transmitter is used to transmit the same data on each antenna where the data is manipulated in some manner differently for each antenna. One example of such an approach is space-time transmit diversity (STTD). In STTD, a first antenna transmits a block of two input symbols over a corresponding two symbol intervals in a first order while at the same time a second antenna transmits, by way of example, the complex conjugates of the same block of two symbols and wherein those conjugates are output in a reversed order relative to how they are transmitted by the first antenna and the second symbol is a negative value relative to its value as an input.
Another approach to improve SINR combines antenna diversity with the need for higher data rate. Specifically, a Multiple-input Multiple-output (MIMO) system with transmit diversity has been devised, where each transmit antenna transmits a distinct and respective data stream. In other words, in a MIMO system, each transmit antenna transmits symbols that are independent from the symbols transmitted by any other transmit antennas for the transmitter and, thus, there is no redundancy either along a single or with respect to multiple of the transmit antennas. The advantage of a MIMO scheme using distinct and non-redundant streams is that it can achieve higher data rates as compared to a transmit diversity system.
Communication system performance demands in user equipment, however, are often dictated by web access. Applications such as news, stock quotes, video, and music require substantially higher performance in downlink transmission than in uplink transmission. Thus, MIMO system performance may be further improved for High-Speed Downlink Packet Access (HSDPA) by Orthogonal Frequency Division Multiplex (OFDM) transmission. With OFDM, multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver, and these tones are termed pilot tones or symbols. These pilot symbols can be useful for channel estimation at the receiver. An inverse fast Fourier transform (IFFT) converts the frequency domain data symbols into a time domain waveform. The IFFT structure allows the frequency tones to be orthogonal. A cyclic prefix is formed by copying the tail samples from the time domain waveform and appending them to the front of the waveform. The time domain waveform with cyclic prefix is termed an OFDM symbol, and this OFDM symbol may be upconverted to an RF frequency and transmitted. An OFDM receiver may recover the timing and carrier frequency and then process the received samples through a fast Fourier transform (FFT). The cyclic prefix may be discarded and after the FFT, frequency domain information is recovered. The pilot symbols may be recovered to aid in channel estimation so that the data sent on the frequency tones can be recovered. A parallel-to-serial converter is applied, and the data is sent to the channel decoder. Just as with HSDPA, OFDM communications may be performed in an FDD mode or in a TDD mode.
One approach to improve spatial diversity of a multipath channel for MIMO communications systems is the vertical BLAST (Bell Laboratories Layered Space Time) or V-BLAST system as shown at FIG. 1. The V-BLAST system uses a vertically layered space-time architecture as described by Wolniansky et al., “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel” (ISSSE, October 1998) and by Wolniansky et al., “Detection algorithm and initial laboratory results using V-BLAST space-time communication architecture” ( IEEE Vol. 35, No. 1, January 1999). The modulation code scheme (MCS) of the V-BLAST circuit includes encoder 100, interleaver 102, and symbol mapper 104. Encoder 100 of the V-BLAST system encodes a serial data stream which is subsequently applied to interleaver circuit 102. The resulting interleaved data is then applied to symbol mapper 104 to produce a desired symbol constellation. The resulting symbols are then applied to serial-to-parallel circuit 106 and transmitted to remote user equipment via antennas 108. The V-BLAST system, therefore, improves communication with a single MCS by dividing a data stream into sub-streams that propagate differently over the wireless channel. The improvement, however, depends on the relative independence of these sub-streams. When there is a high correlation between the sub-streams, data may not be properly detected at the remote user equipment.
A further improvement over the V-BLAST system is shown in the per antenna rate control (PARC) circuit of FIG. 2. The PARC circuit includes four separate MCS circuits. Each MCS circuit includes an encoder 200, and interleaver 202, and a symbol mapper 204. The serial-to-parallel circuit 206 divides a data stream into four separate sub-streams. Each sub-stream is applied to a respective encoder circuit 200. Each encoder preferably allocates a different data rate according to the channel quality of each corresponding transmit antenna 208. For CDMA applications, each encoder circuit 200 may also multiply each sub-stream by a spreading code corresponding to the intended user equipment. The encoded sub-streams are subsequently interleaved, symbol mapped, and transmitted over transmit antennas 208. The PARC system, therefore, improves communication by dividing a data stream into sub-streams that propagate differently over the wireless channel and allocating specific data rates to each sub-stream corresponding to the quality of the respective wireless channel. The improvement, however, significantly increases signal processing complexity. A separate MCS circuit is required for each respective transmit antenna. Moreover, remote PARC receivers must identify and report the SINR for each transmit antenna.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements may be made, including by addressing some of the drawbacks of the prior art. In particular, embodiments of the present invention improve communication quality and significantly reduce signal processing complexity compared to the PARC system. Some of these issues are described in co-pending U.S. patent application Ser. No. 10/230,003, filed Aug. 28, 2002, entitled, “MIMO HYBRID-ARQ USING BASIS HOPPING”, and incorporated herein by reference. In this referenced application, multiple independent streams of data are adaptively transmitted with a variable basis selected to improve signal quality. Further, a receiver is provided that decodes the transmitted signals including the multipaths therein. While this improvement therefore provides various benefits as discussed in the referenced application, the inventors also recognize still additional benefits that may be achieved with such systems. Accordingly, the preferred embodiments described below are directed toward these benefits as well as improving upon the prior art.